Imidazolinone compounds, for instance, those described in U.S. Pat. No. 4,188,487, U.S. Pat. No. 4,798,619 and U.S. Pat. No. 5,334,576, are highly potent, broad spectrum, environmentally benign, herbicidal agents. Although said imidazolinones demonstrate excellent herbicidal activity, when used as the sole active ingredient they do not always achieve effective control of the full spectrum of weed species encountered in commercial agronomic practice. Such gaps in the spectrum of control can often be remedied by co-treatment with another herbicide known to be effective against the relevant weed species. For imidazolinone resistant or tolerant crops, efficacy over a broad spectrum of weed species remain a challenge. Accordingly, there is ongoing research to create more effective imidazolinone-based broad spectrum herbicidal combinations for said crops.
Combinations of AHAS-inhibiting herbicides plus growth regulator herbicides are described in U.S. Pat. No. 5,739,080. However, U.S. Pat. No. 5,739,080 discloses the use of said combination in a method for the reduction of phytotoxicity to crop plants, particularly grass crop plants such as sorghum, corn and wheat. Although reduction in plant injury is exemplified for the combination of an AHAS-inhibiting herbicide such as a sulfonyl urea plus a growth regulator herbicide such as the benzoic acid type (e.g., dicamba) or the phenoxy acid type (e.g., 2,4-D and MCPA), no biological interaction such as antagonism or synergism is demonstrated for the combination of an imidazolinone herbicide plus MCPA. Further, no indication or anticipation of synergism is disclosed.
Therefore, it is an object of this invention to provide a method for the synergistic control of a broad spectrum of undesirable broadleaf and grass plant species.
It is another object of this invention to provide a synergistic herbicidal composition useful for broad spectrum weed control.
It is a feature of this invention that the inventive synergistic herbicidal methods and compositions may be applied in the presence of imidazolinone-resistant or imidazolinone-tolerant crops.
The present invention provides a method for the synergistic control of undesirable plants which comprises applying to the locus of said plants or to the foliage or stems of said plants a synergistically effective amount of a combination of an imidazolinone herbicide plus (4-chloro-1-methylphenoxy)acetic acid (MCPA).
The present invention also provides a synergistic herbicidal composition which comprises an agriculturally acceptable carrier and a synergistically effective amount of a combination of an imidazolinone herbicide plus MCPA.
Imidazolinone compounds such as those described in U.S. Pat. No. 4,188,487; U.S. Pat. No. 4,798,619 or U.S. Pat. No. 5,334,576, are highly potent, broad spectrum, environmentally benign, herbicidal agents. Although said imidazolinones demonstrate excellent herbicidal activity when used as the sole active ingredient, they do not always achieve effective control of the full spectrum of weed species encountered in commercial agronomic practice. The widespread use of broadleaf herbicides in small grain crops has led to a shift in weed species so that effective control of grass weeds has become a significant problem in agronomic practice. Effective control of both broadleaf and grass weed species with a single herbicide typically requires high use rates and multiple applications, a practice which commonly leads to weed resistance.
Surprisingly, it has now been found that a combination which comprises an imidazolinone herbicide plus (4-chloro-2-methylphenoxy)acetic acid, (MCPA) demonstrates synergistic control of a broad spectrum of undesirable broadleaf and grass plant species. That is, the application of the combination of the invention gives a mutual reinforcing action such that the application rates of the individual herbicidal components can be reduced and still the same herbicidal effect is achieved or, alternatively, the application of the combination of herbicidal components demonstrates a greater herbicidal effect than that which can be expected from the effect of the application of the individual herbicidal components when applied singly at the rate at which they are present in the combination (synergistic effect), The combination of this invention is particularly suited for use in imidazolinone-resistant or imidazolinone-tolerant crops, preferably a cereal crop, more preferably wheat.
Imidazolinone herbicides suitable for use in the combination of the invention include compounds of formula I 
wherein
Y and Z are each independently H, C1-C4alkyl optionally substituted with one C1-C4alkoxy group or Y and Z may be taken together to form a group xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94.
Preferred imidazolinone compounds of formula I are:
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)nicotinic acid (imazapyr);
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-3-quinolincarboxylic acid (imazaquin);
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-methylnicotinic acid (imazapic;
5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)nicotinic acid (imazothapyr): or
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-methoxymethyl)nicotinic acid (imazamox).
More preferred compounds of formula I are:
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-(methoxymethyl)-nicotinic acid (imazamox);
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-methyinicotinic acid (imazapic);
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)nicotinic acid (imazapyr); or a mixture thereof.
Especially preferred compounds of formula I are:
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-(methoxymethyl)-nicotinic acid (imazamox);
2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-methyinicotinic acid (imazapic);
or a mixture thereof.
In the specification and claims, the term MCPA designates the auxin-type herbicide, (4-chloro-2-methylphenoxy)acetic acid, as well as its salts, alkyl or aryl amides, alkylesters, and/or alkyl thioesters. Preferably, MCPA designates (4-chloro-2-methyl-phenoxy)acetic acid, its sodium, potassium, or dimethylamine salt, its amide, t-chloroanilide, C1 to C10-alkyl ester and/or C1 to C4-alkylthio ester. More preferably, MCPA designates (4-chloroz-2-methylphenoxy)acetic acid, its sodium, potassium or dimethylamine salts, its amide, 2-chloroanilide, isobutyl ester, isooctyl ester and/or ethyl thioester. Most preferably, MCPA designate (4-chloro-2-methylphenoxy)acetic acid and/or its isooctyl ester.
(4-Chloro-2-methylphenoxy)acetic acid, its sodium, potassium, or dimethylamine salts, its amide, 2-chloroanilide, isobutyl ester, isooctyl ester and ethyl thioester are herbicides known from Farm Chemical Handbood ""99, Meister Publishing Company 1999, p. Ci 246, C246, and Ci 303.
The synergistic herbicidal combination of the invention comprises an imidazolinone herbicide, preferably an imidazolinone of formula I, more preferably imazamox, imazapic, imazapyr, or a mixture thereof plus MCPA. Synergistic herbicidal combinations suitable for use in the method of the invention include those wherein the weight/weight ratio of imidazolinone to MCPA is about 1:4 to 1:20, especially about 1:10 to 1:20.
Thus, in accordance with the method of invention a synergistically effective amount of a combination of an imidazolinone herbicide plus MCPA is applied to the locus, foliage or stems of undesirable plants, optionally in the presence of an imidazolinone-resistant or imidazolinone-tolerant crop, preferably a cereal crop, more preferably wheat.
In actual practice, the combination of the invention may be applied to the plant stem or foliage or the locus thereof in the form of an aqueous concentrate, or water dispersible granule, a wettable powder, a soluble granule, or in any form conventionally used in agronomic practice.
The present invention also provides a synergistic herbicidal composition which comprises an agriculturally acceptable carrier and a synergistically effective amount of a combination of an imidazolinone herbicide plus MCPA. The agriculturally acceptable carrier may be a solid or a liquid, preferably a liquid, more preferably water. While not required, the synergistic herbicidal composition of the invention may also contain other additives such as fertilizers, inert formulation aids, i.e. surfactants, emulsifiers, defoamers, dyes, extenders or any of the conventional inert ingredients typically employed in herbicidal formulated products.
Imidazolinone herbicides suitable for use in the composition of the invention include compounds of formula I 
wherein
Y and Z are each independently H, C1-C4alkyl optionally substituted with one C1-C4alkoxy group or Y and Z may be taken together to form a group xe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94.
Preferred synergistic herbicidal compositions of the invention are those compositions wherein the imidazolinone herbicide is imazamox, imazapic, imazapyr, imazethapyr, imazaquin or a mixture thereof. More preferred compositions of the present invention are those wherein the imidazolinone herbicide is imazamox, imazapic, imazapyr or a mixture thereof. Especially preferred compositions of the invention are those wherein the imidazolinone herbicide is imazamox or imazapic or a mixture thereof.
The synergistically effective amount of the combination suitable for use in the composition of the invention may vary according to prevailing conditions such as weed pressure, application timing, mode of application, weather, soil conditions, topographical character, target crop species and the like. In actual practice, application rates sufficient to provide about 0.10 kg/ha to 1.00 kg/ha, preferably about 0.25 kg/ha to 0.75 kg/ha of active ingredients are suitable.
In another embodiment of the present invention the application rate of the imidazolinone herbicide or mixture thereof is in the range of 1-200 g/ha, preferred 10-150 g/ha, more preferred 15-50 g/ha.
In another embodiment of the present invention the application rate of MCPA is in the range of 50-950 g/ha, more preferred 150-700 g/ha, especially preferred 200-400 g/ha.
For a more clear understanding of the invention, specific examples thereof are set forth below. These examples are merely illustrative, and are not to be understood as limiting the scope and underlying principles of the invention in any way.
In the following examples, synergism is determined by the Colby (Colby, S. R., Weeds, 1967(15), p. 20-22) method, i.e. the expected (or predicted) response of the combination is calculated by taking the product of the observed response for each individual component of the combination when applied alone divided by 100 and subtracting this value from the sum of the observed response for each component when applied alone. Synergism of the combination is then determined by comparing the observed response of the combination to the expected (or predicted) response as calculated from the observed responses of each individual component alone. If the observed response of the combination is greater than the expected (or predicted) response then the combination is said to be synergistic.
The foregoing is illustrated mathematically herein below, wherein a two-way combination, C, is composed of component X plus component Y and Obs. designates the observed response of the combination C.             (              X        +        Y            )        -          XY      100        =      Expected    ⁢          xe2x80x83        ⁢    response    ⁢          xe2x80x83        ⁢          (              Exp        .            )      